A spark plug used for ignition in an internal combustion engine such as an automobile engine has a tendency toward increase in temperature inside a combustion chamber for the purposes of increase in engine output and improvement in fuel efficiency. An engine of the type having a combustion chamber into which a discharge portion of a spark plug is protruded while located opposite to a spark discharge gap of the spark plug has been used widely in order to enhance ignitability. Under such circumstances, the discharge portion of the spark plug is apt to be worn out acceleratedly by spark discharge because it is exposed to a high temperature. In order to improve spark resistance of the discharge portion opposite to the spark discharge gap, a large number of proposals have been made for a spark plug of the type in which a precious metal tip mainly containing Ir or the like is welded to a leading end of an electrode.
For example, in Patent Document 1, a precious metal tip containing Ir as a main component, Rh and Ni has been described. This is for the following reasons. Good durability against abrasion caused by spark discharge (hereinafter also referred to as spark abrasion) can be used wisely since Ir has a high melting point (2410° C.). In addition, resistance to abrasion caused by volatilization of Ir oxidized at a high temperature (of not lower than 900° C.) (hereinafter also referred to oxidation abrasion) can be improved since Rh is added to Ir in order to prevent the oxidation abrasion of Ir. In the spark plug having such a discharge portion made of a precious metal tip containing Ir as a main component, and Rh, however, abnormal abrasion still occurred in accordance with the condition of use so that an outer circumferential side surface of the discharge portion which was not a discharge surface located in a position opposite to the spark discharge gap was gouged out circularly after operation. In Patent Document 1, therefore, attention has been paid to Ni as a component for suppressing the abnormal abrasion. That is, since Ni was further added to the precious metal tip containing Ir as a main component, and Rh, the abnormal abrasion could be suppressed while both spark abrasion and Ir oxidation abrasion could be prevented.
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-359050